1. Field of the Invention
This invention relates to imaging apparatus. More particularly, the invention relates to focal plane shutter systems.
2. Related Art
Fast shutter speeds in cameras that expose onto a large imaging medium are typically obtained with focal plane shutters. A typical application for focal plane shutters is in airborne reconnaissance. Military reconnaissance cameras collect continuous, overlapping frames of imagery from a moving aircraft. Reconnaissance missions require exposure times as short as {fraction (1/5000)}th of a second. High frame repetition rates, from one to six exposures per second, are needed in airborne reconnaissance to obtain continuous imagery.
The imaging medium used with a focal plane shutter may be film or any form of light-sensitive electronic detector, such as a charge coupled device. Imaging media ranging in size from 25 mm by 35 mm up to 9 inches square are typical. Conventional focal plane shutters achieve exposure by moving a slit shaped opening over the imaging medium. The slit, generally rectangular, is moved in the direction of its width. The slit is long enough to expose the imaging medium completely. When the slit completes its traversal across the medium, full exposure is achieved.
The exposure time of a focal plane shutter is determined by the ratio of slit width to slit traversal speed. For example, a desired exposure time of {fraction (1/3000)} of a second using a 0.1 inch wide slit requires a constant imaging medium traversal speed of 300 inches per second. Accurate exposure time is an important consideration in reconnaissance and other applications. Reliability of the focal plane shutter is also of critical importance, particularly in military applications. High acceleration forces are needed to move a slit from an initial resting position up to a fast slit speed. The required forces create wear and tear in the shutter mechanism. This causes reliability problems in conventional focal plane shutters after extended use. Conventional cameras which operate at high frame repetition rates (more than one frame per second) for extended periods require frequent repair or replacement of the shutter.
Some conventional focal plane shutters use a complex dual belt arrangement to achieve multiple slit widths. An example of this is described in U.S. Pat. No. 3,179,029 to C. Vinton et al., Apr. 20, 1965. In such shutter mechanisms, two belts are driven in constant rotation past an imaging medium. The belts each have one or more slits. Exposure requires that a slit in one belt overlap with a slit in the other belt. This configuration admits light to the imaging medium when the overlapped slits pass over the medium. By controlling the degree of overlap of the slits, the exposure is controlled. Focal plane shutters of this type have problems with accuracy and reliability. When slit widths must be narrow (less than 0.1 inch), the inability to accurately position multiple belts relative to each other can cause errors in slit overlap, leading to errors in exposure. Moreover, collision between slit edges during reset operation is a common cause of failure in conventional designs.
Another problem with conventional overlapping belt shutter systems is that too much time is required to switch from a long exposure to a short exposure. During the time taken to adjust the amount of slit overlap, conventional shutter systems are unable to continue imaging. For imaging applications such as military reconnaissance, continuous imaging at high repetition rates is essential.
Hence there is a need for a focal plane shutter system that allows rapid changes in exposure, permits precise control of exposure, and is not prone to excessive wear.
According to the present invention, a system and method are provided for exposing an imaging medium. The method uses a focal plane shutter system with two reversible motors controlled by a servo control processor. A belt connects the motors. One end of the belt is attached to a shaft of the first reversible motor. The other end of the belt is connected to a shaft of the second reversible motor. In the belt, there are a plurality of fixed width slits. The servo control processor selects one of the fixed width slits according to a desired exposure time setting. The two reversible motors move the belt during an imaging cycle. During the imaging cycle, the first motor winds the belt around its shaft. The second motor simultaneously unwinds the belt from around its shaft while controlling belt tension. The servo control processor controls the motors to accelerate the selected slit up to a constant imaging medium traversal speed. The constant traversal speed is also determined by the desired exposure time setting. The selected slit is traversed across the imaging medium at the constant imaging medium traversal speed. After fully exposing the imaging medium, the selected slit is decelerated to a resting position. Motor direction is reversed for both motors in the next imaging cycle. The sequence of accelerating the selected slit, traversing the imaging medium at constant speed, decelerating the selected slit, and reversing the motors, is repeated until a new slit is selected.
The present invention provides solutions to the problems associated with conventional focal plane shutter systems and methods.
The invention advantageously avoids the reliability problems associated with wear and tear in conventional shutter mechanisms. This is accomplished by using several fixed width slits. The present invention selects an appropriate slit to move back and forth past the image medium. Slits are selected to lessen the high acceleration forces which cause reliability problems in conventional systems. At the same time, the present invention is able to provide increased accuracy over conventional systems which use overlapping belts to vary slit width. Rather than using overlapping belts, the present invention uses fixed width slit selection. Therefore, the present invention does not suffer any loss in accuracy due to difficult belt alignment problems as do conventional systems. Since the present invention is accomplished using only a single belt, conventional problems caused by collision between slit edges are avoided. This adds to the improved reliability of the present invention over conventional systems.
Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings.